There are a number of known methods to eliminate or to work around video surveillance camera position errors. Typically, to reduce the likelihood of a positioning error, components of a video camera assembly or system are often manufactured in such a precise manner that any component tolerance or assembly errors do not significantly impact the accuracy of the dome. For instance, a camera block in the assembly can have some play, an image sensor mounted inside the camera block can be offset from the center of the lens, the gear-train can have inconsistencies, the homing sensors can have some mounting tolerances, the dome housing mounting mechanism can have fabrication tolerances, and finally the structure that the customer mounts the dome to can have some play. While such inaccuracies and assembly tolerances may have been acceptable in the past, several evolutionary factors in surveillance systems demand accuracy improvements. These advances require tighter tolerances as well as better performance repeatability from one similar component to the next.
In particular, video surveillance systems have progressed over the years from 1×-10× zoom lenses to present-day lenses having typical zoom capability of 1×-35× optical and up to 420× digitally. Given the dramatic increase in zooming capability, any error in positioning, whether originating in the manufacturing, assembly or installation of a particular component, is magnified to the point of making the system unusable. For example, at 35× zoom, a 1 degree error in the positioning of a surveillance camera will cause an offset of more than a half of the field of view. In this example, a 1 degree error would put whatever object or region that should have originally been at the center of the viewing field, just off the edge and out of view. Adding a 16× digital zoom capacity further compounds the problem.
Moreover, in previous systems, it was typically only necessary to have performance or positioning repeatability when establishing the home position. If a video surveillance camera were replaced, the operator would subsequently have to reload any previously identified targets, patterns, privacy zones, etc. However, increasingly advanced video surveillance products offer the capability to store the pan/tilt/zoom coordinates of the previously identified targets, patterns, privacy zones, etc. in a non-volatile memory in an I/O base or other device in the system which is external to the video dome or camera. As such, if a video camera needs to be replaced, the replacement camera can load all the setup data from the external memory device, thereby reducing operator overhead and installation/setup time to restore the previous functionality of the system. Still, in such a system, the replacement video cameras not only have to home with consistent, repeatable results, but they must also operate and perform substantially accurately and similarly from one camera to the next. As noted above, replacing a camera with one that operates with even 1 degree of difference due to equipment tolerances can result in the desired image being out of the field of view.
Factory calibration could potentially compensate for all but field-related installation accuracy problems. For example, a factory calibration could store offsets in pan and tilt to compensate for errors detected in home position. However, this would require dedicated personnel at the factory or repair center, and the associated cost, to calibrate every video camera assembly to a high degree of accuracy, even though the increased level of accuracy is needed only when one video camera is replacing another in a system having many targets, patterns, privacy zones and other specialized settings with a high zoom level.
In view of the above, it is desirable to provide an on-site, low cost field alignment system and method that allows a replacement video camera to compensate for camera-to-camera alignment variations, including tolerance or shifting of the mounting base, platform or other field related alignment issues.